Conures, Lories, Senegals, Ring Necks, Caiques, Mini Macaws, Pionus, Quakers, Small Cockatoos and similar sized birds will generally need medium parrot cages to be comfortable.
If you have more than one bird, you obviously need to go to a larger cage to give the birds plenty of room. Generally speaking, the more room your birds have, the happier they will be.